The demands of aerospace and defense has been one of the main driving forces for fast development of infrared (IR) optics, in particular for infrared optics that will operate in a short-wave infrared range (SWIR) of 1-3 μm, a middle-wave infrared range (MWIR) 3-5 μm, and long-wave infrared range (LWIR) of 8-14 μm. The materials that can be used for the LWIR region are limited because the most frequently materials that are used are oxide materials and they are not transparent in the LWIR spectral regime. Germanium (Ge), zinc-selenide (ZnSe) and zinc-sulfide (ZnS) are the most popular optical window materials for use in the LWIR regime, these materials having a transmittance of 47%, 71% and 75%, respectively. An additional consideration of importance is that the IR optic made of the foregoing materials may be exposed to severe environmental condition for various applications. Consequently, environmentally durable antireflection (AR) coatings are necessary for LWIR optics applications.
The optical performance of an AR coating is dominated by the refractive index of outermost layer. A low refractive index of the outermost layer enables one to achieve a broadband AR coating. However, coating durability and environmental stability are mainly affected by the outermost layer in optical coatings. As a result, the material property of the outermost layer plays a critical role not only in optical performance, but also in mechanical strength and environmental stability. An IR-AR coating from 7.7μ to 10.3μ has been established where ytterbium fluoride (YbF3) is used as the outermost layer. There are IR-AR coatings in current use that pass the both the optical specification and a moderate abrasion test with a minimum bearing force of 1 pound. However, recently it has been indicated that a severe abrasion test will be required for future LWIR AR coated products. The existing AR coating, however, could not pass the severe abrasion test with a barring force between 2 and 2.5 lbs (MIL-C-48497A). Consequently, there is a need for a low refractive index coating material with durable mechanical property to ensure both a broadband antireflection spectral performance and to withstand the severe abrasion test. At the present time there are no optical AR coatings that give satisfactory performance, transmission, including abrasion resistance, in all of (1) a short-wave infrared range (SWIR) of 1-3 μm, (2) a middle-wave infrared range (MWIR) 3-5 μm, and (3) a long-wave infrared range (LWIR) of 8-14 μm.